Laser using phosphate base laser glass

ABSTRACT

A laser device uses a material for incorporating into a glass laser host consisting essentially of a phosphate glass composed principally of an erbium metaphosphate admixed with an ytterbium metaphosphate. The properties of the glass may be varied by additional R(PO3)3 and S(PO3)2 contents wherein said R is a trivalent ion selected from the group consisting of aluminum, lanthanum, gadolinium, gallium, scandium, yttrium, neodymium, cerium, europium, manganese, molybdenum and chromium, and said S is a divalent ion selected from the group consisting of zinc, magnesium, cadmium, calcium, strontium or barium. The resulting glass composition provides a material capable of producing laser emission from the erbium ion at a wavelength of 1.536 microns, when disposed in a resonant cavity having an output reflector whose reflectance is a maximum of 80 percent.

United States Patent 1 Snitzer et a1.

LASER USING PHOSPHATE BASE LASER GLASS [75] Inventors: Elias Snitzer,Wellesley; Robert E.

Grai, Southbridge, both of Mass.

[73] Assignee: American Optical Corporation, Southbridge, Mass.

[22] Filed: Nov. 10, 1971 [211 App]. No.: 197,394

Related US. Application Data [63] Continuation-impart of Ser. No.80,458, Oct. 13,

1970, abandoned, which is a continuation of Ser. No.

712,744, March 13, 1968, abandoned.

[52] US. Cl. ..33l/94.5, 330/43 [51] Int. Cl. ..Hls 3/16 [58] Field ofSearch ..33 l/94.5; 330/43;

[56] References Cited UNITED STATES PATENTS 3,250,721 5/1966 Paolis..33l/94.5 3,533,956 /1970 Snitzer ..331/94.5

OTHER PUBLICATIONS Snitzer e t al Phosphate Glass Er V Laser. IEEE J.

3,731,226 May 1, 1973 Primary Examiner-William L. Sikes AttorneyWilliamC. Ncalon et al.

[57] ABSTRACT A laser device uses a material for incorporating into aglass laser host consisting essentially of a phosphate glass composedprincipally of an erbium metaphosphate admixed with an ytterbiummetaphosphate. The properties of the glass may be varied by additionalMP0,), and S(PO contents wherein said R is a trivalent ion selected fromthe group consisting of aluminum, lanthanum, gadolinium, gallium,scandium, yttrium, neodymium, cerium, europium, manganese, molybdenumand chromium, and said S is a divalent ion selected from the groupconsisting of zinc, magnesium, cadmium, calcium, strontium or barium.The resulting glass composition provides a material capable of producinglaser emission from the erbium ion at a wavelength of 1.536 microns,when disposed in a resonant cavity having an output reflector whosereflectance is a maximum of percent.

5 Claims, l Drawing Figure LASER USING PHOSPHATE BASE LASER GLASSCROSS-REFERENCE TO RELATED APPLICATIONS This application is acontinuation-in-part of our previously filed application, Ser. No.80,458, filed Oct. 13, 1970, which is now abandoned, and which, in turn,was a continuation of our previously filed application, Ser. No.712,744, filed Mar. 13, 1968, which is now abandoned.

BACKGROUND OF THE INVENTION or core composed of a so-called lasermaterial. With a laserable quantity of erbium ions in a phosphate baseincorporated into a glass laser host, laser action from the erbium ionsoccurs at a wavelength of 1.536 microns. Although it is possible toobtain laser action at liquid nitrogen temperatures, in a phosphateglass formed solely of P and Er O as batch constituents, greater laserefficiency can be obtained by using a laser 0 ion of erbium and aso-called sensitizing ion in the wavelength to be reported. In asimilarly doped silicate base glass laser, for example, the erbium laseremission is at 1.543 microns.

SUMMARY This invention relates generally to laser devices and moreparticularly to a novel composition for an erbium doped glass laser. Thecomposition is essentially an erbium metaphosphate. In many applicationsof erbium lasers it is important to have laser action occur at as shorta wavelength as possible, as for example in a range finder. By providinga phosphate base glass host for erbium, laser action occurs at 1.536microns when utilized in a resonant cavity, the output reflector ofwhich has a maximum reflectance of 80 percent. In ad dition, it has beenfound that by providing a phosphate base, a glass having a lowcoefficient of thermal expan sion is produced.

Therefore, it is an object of this invention to provide a host materialand resonant cavity configuration for the erbium ion with resultinglaser emission for erbium at 1.536 microns.

A further object of this invention is to provide a laserable materialhaving a low coefficient of expan- SlOll.

BRIEF DESCRIPTION OF THE DRAWING The single FIGURE of the drawing is agraph showing the fluorescence spectra for trivalent erbium in phosphateand silicate base glasses at room temperature.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Lasers, otherwise referred toas optical masers, are light-amplifying or light-producing devices andare specifically adapted to provide an output of high intensity,coherent monochromatic light. Such light is produced in a laser (anacronym for light amplification by stimulated emission of radiation) byphotonic emission from the so-called active atoms or ions of a body hostmaterial to help pump the laser ion. In this invention an ytterbium ionis used as the sensitizing ion to transfer energy from a flash tube tothe erbium ion. This teaching is fully disclosed in U. S. Pat.application Ser. No. 420,280, assigned to the assignee of the instantapplication and forms no part of the present invention. However, theteachings contained in said application are incorporated into thepresent disclosure. In that application, trivalent ytterbium is pumpedby a flash tube from an initial ground level to a higher level with theground level designated as P and the upper level P in spectroscopicnotation. An energy transfer then takes place from the upper level ofthe trivalent ytterbium to the upper I of trivalent erbium. Anonradiative transition then occurs from the I level to the 1, level oftrivalent erbium. Subsequent transitions occur between energy levels 1,and 1, producing laser output. By providing a phosphate base glasscomposition in the concentrations that will be later explained, thelaser emission takes place at 1.536

microns.

Preparation of the laser glass can be understood with reference to thefollowing chemical reaction:

In one embodiment of the invention, an erbium-ytterbium metaphosphateglass which produces laser emission at 1.536 microns, was prepared fromthe following batch constituents:

no, 52% no 47.5% E130, 0.5%

Calculations have shown that there can be a deviation from thestoichiometry of the equations:

In addition to Er O and Yb,0 the trivalent constituents may also includeone or more oxides from the group consisting of aluminum, lanthanum,gadolinium,

gallium, scandium, yttrium, neodymium, cerium, eu-

ropium, manganese, molybdenum and chromium, while still producing laseremission from erbium at 1.536 microns. A further constituent, one ormore oxides of a divalent ion, SO, wherein S is selected from the groupconsisting of zinc, magnesium, cadmium, calcium, strontium and barium,may also be introduced into the batch. On heating, said SO content willproduce a metaphosphate according to the following chemical reaction:

The addition of the above SO content to the batch has been found tochange the physical properties of the glass without changing thewavelength at which the erbium will lase. Accordingly, aluminum oxidewithin the range of 20 weight percent may be introduced into the batch.The addition of aluminum oxide increases the glasss durability andstability and prevents crystallization or devitrification of the glass.In order to increase the fluxing properties of the glass, a zinc oxidecontent within the range of approximately 0-20 weight percent may alsobe introduced into the glass. A glass having the following batchcomposition was prepared:

Weight Ago, 8.44% P 0 61.30% ZnO 14.66% Nd O 20% Er,0 .50% Yb O 14.90%

The glass prepared from the above batch constituents had an index ofrefraction of 589 nm of 1.52, a density of 2.95, and an expansioncoefficient of 4.2 X /ALC.

Experimentally it has been determined that the concentration of erbiumions necessary for laser action is between the range of 10 ions percubic centimeter and 3 X 10 ions per cubic centimeter. Expressed in aweight percent, this range would be approximately between values of001-30 weight percent of the oxide of Er o Said range of erbium oxide ina phosphate base glass host contains a laserable quantity of erbiumions, and when erbium ions in said range in a phosphate base areincorporated within a glass laser host and pumped by an energy sourcesuch as a flash tube, a sufficient inversion in population between twoenergy levels results so as to provide enough gain in the laserwavelength of stimulated emission to overcome all light losses in saidlaser host material. The range for the concentration of ytterbium ionsrequired for energy transfer to the erbium ions is between the values ofapproximately 0.0155 weight percent of the oxide, but the total Er O andYb O batch content cannot exceed approximately 70 weight percent. Theefficiency of the glass can also be improved by more than a factor of 3,by the addition of small amounts of neodymium oxide (Nd O It has beendetermined experimentally that the optimum Nd,0 concentration is about0.2 weight percent. Neodymium not only sensitizes the fluorescence oferbium but also quenches it, which is evident by the decrease inlifetime from 9.4 milliseconds for glass with a 0.5 weight percent Er Oweight percent Yb,o, and no M1 0, to 8.2 milliseconds for the additionof 0.2 weight percent of Nd o Erbium-phosphate laser glass can besuccessfully prepared from a batch having a P 0 constituent within therange of approximately 30-90 weight percent.

The zinc oxide referred to earlier within said range of approximately0-20 weight percent may be replaced by oxides of magnesium, cadmium,calcium, strontium and barium in a mole percent range corresponding tothe mole percent range of approximately 0-20 weight percent of zincoxide. Accordingly, said oxides of magnesium, cadmium, calcium,strontium and barium within the range of approximately 0-32 mole percentmay be introduced into the batch.

The aluminum oxide content within the range of approximately 0-20 weightpercent may be replaced by oxides of lanthanum, gadolinium, gallium,scandium, yttrium, neodymium, cerium, europium, manganese, molybdenumand chromium within the range of approximately 0-10 weight percent ofthe oxide.

Although we are, at present, uncertain as to the exact reason why aphosphate base host for erbium produces laser action at 1.536 microns,tests have indicated that with a phosphate base, laser action from theerbium ions occurs only at that wavelength when the output reflector forthe resonant cavity has a reflectance of percent or less. Referring tothe drawing, there is shown the fluorescent spectra for trivalent erbiumin a phosphate and in a silicate base glass. The spectrum for thephosphate base glass was obtained with an output mirror whosereflectance is approximately 60 percent which appears to be the idealvalue. The silicate curve shows a peak labelled A at 1.536 microns and apeak labelled B at 1.543 microns. The phosphate curve also showscorresponding peaks labelled A at 1.536 microns and labelled B at 1.543microns. If intensity were the only factor to consider, laser actionwould take place at the wavelength at which the fluorescent intensity isgreatest. In this case, that is the A intensity for the silicate and Afor the phosphate, both of which are 1.536 microns. There is, however,another consideration. Laser emission tends to occur at the longer oftwo wavelengths. This effect is the result of the fact that the terminalstate of the longer wavelength transition is slightly above the groundstate. In the silicate base, the ratio of B'lA intensities is morenearly equal to one than is.the ratio of B/A intensities in thephosphate base. The result is that laser action from the erbium ions ina silicate base occurs in the B line of 1.543 microns, whereas laseraction from the erbium ions in a phosphate base occurs at 1.536 microns,that is, the A peak.

The drawing shows further peaks at C and C. At liquid nitrogentemperature of 77K there is no emission in these peaks. Above roomtemperature the peak indicated as C for the silicate glass increasesmore rapidly with temperature than does the corresponding peak labelledC for the phosphate glass. The net result is that in the silicate baseat a temperature approximating that of boiling water, laser emissiondiscontinuously shifts from a value of 1.543 microns to approximately1.57 microns. However, a phosphate base erbium laser does not show thisinstability and the wavelength of laser emission is stable at 1.5 36microns and is independent of temperature up to C.

The short wavelength of laser emission and temperature independence arevery important features for an erbium laser. At present, an importantuse for an erbium laser is in a range finder. A range finder is aninstrument that measures distance by determining the time interval ittakes for a signal sent out to reach an object and reflect back to adetector. A range finder operates by the principle upon which radar inthe microwave region operates. A range finder requires a Q-switchedpulse and detector which is fast enough to follow the short durationQ-switched emission and a response time of a few nanoseconds. Atpresent, the detector which appears to be most promising at thiswavelength is a germanium diode. However, this detector shows a rapidlychanging response characteristic in this wavelength region. The netresult is that the efflciency for'detecting radiation at 1.536 micronsis better than detecting radiation at 1.543 microns by a factor of about20 percent. The increase in detection efficiency leads to acorresponding decrease in the energy necessary for a given application.Also, if a laser discontinuously shifts from emission at 1.543 micronsout to 1.57 microns, two problems are encountered. First, the detectionefficiency is drastically reduced at the longer wavelength. Secondly,since it is desirable to use a narrow wavelength filter to discriminateagainst background light, the filter width would have to be wide enoughto permit both the 1.543 and 1.57 micron lines to pass. Widening thefilter would lead to a corresponding increase in the background light.These two problems are so undesirable in range finder applications thatit is necessary to incorporate some means of temperature control whichis extremely costly.

However by providing a phosphate base erbium laser in accordance withthis invention, there is no discontinuous shifting of emissionwavelength as the temperature increases.

We claim: v 1. A laser device which uses a rod of glass material as hostfor the active ions, the rod being disposed in a resonant cavity, theimprovement in which comprises the laser glass material consistingessentially of a laserable quantity of erbium ions in a phosphate baseglass, ytterbium ions being present as sensitizer ions, theconcentration of erbium ions being present in the range of approximately0.01 to 30 weight percent on an oxide basis, ytterbium ions beingpresent in a concentration in the range of approximately 0.01 to 55weight percent on an oxide basis, total erbium and ytterbium on an oxidebasis being less than approximately 70 weight percent, and the outputreflector having a maximum reflectance of 80 percent thereby providinglaser emission at a wavelength of 1.536 microns.

2. A laser device according to claim 1 wherein the glass material has abatch constituent of 52.0 weight percent P 0 47.5 weight percent Yb Oand 0.5 weight percent F1 0,.

3. A laser device according to claim 1 wherein the glass materialincludes an A1 0 content within the range of approximately 0-20 weightpercent and ZnO content within the range of approximately 0-20 weightpercent.

4. A laser device according to Claim 1 wherein the glass material hasthe following batch constituents:

A1 0, 8.44 weight percent 1 ,0 61.30 weight percent ZnO 14.66 weightpercent Nd,0, .20 weight percent E 0, .50 weight percent Yb,0, 14.90weight percent 5. A laser device according to claim 1 furthercharacterized by the inclusion within the glass material of a divalentmetal oxide within the range of approximately O-32 mole percent whereinsaid divalent ion is selected cent.

2. A laser device according to claim 1 wherein the glass material has abatch constituent of 52.0 weight percent P2O5, 47.5 weight percentYb2O3, and 0.5 weight percent Er2O3.
 3. A laser device according toclaim 1 wherein the glass material includes an Al2O3 content within therange of approximately 0-20 weight percent and ZnO content within therange of approximately 0-20 weight percent.
 4. A laser device accordingto Claim 1 wherein the glass material has the following batchconstituents: Al2O38.44 weight percent P2O561.30 weight percent ZnO14.66weight percentNd2O3.20 weight percentEr2O3.50 weight percentYb2O314.90weight percent
 5. A laser device according to claim 1 furthercharacterized by the inclusion within the glass material of a divalentmetal oxide within the range of approximately 0-32 mole percent whereinsaid divalent ion is selected from the group consisting of zinc,magnesium, cadmium, calcium, strontium, and barium and mixtures thereof,and a trivalent oxide within the range of approximately 0-10 weightpercent, said trivalent ion being selected from the group consisting oflanthanum, gadolinium, gallium, scandium, yttrium, neodymium, cerium,europium, maganese, molybdenum, and chromium and mixtures thereof, and aP2O5 content within the range of approximately 30-90 weight percent.